Once the nuclear reactions stop, the star's center begins to cool. The plasma slows down, its outward push growing weak. The balance is broken. The plasma collapses in on itself.

—A Black Hole Is NOT a Hole,
Carolyn Cinami DeCristofano

Which cause does the text describe?
The center begins to cool.
The nuclear reactions stop.
The plasma collapses in.
The push grows weak.